Originally published in The Arizona Daily Star, 2000.
Tucsonans to lead 2 probes' photo flyby

Tate Williams

Tucson scientists are in charge of capturing images of Jupiter as two robotic spacecraft converge at the planet this month for the first time in history.

As the veteran Galileo spacecraft spends its last years orbiting around Jupiter and the newcomer Cassini spacecraft darts past the giant planet, the two are performing several experiments and snapping thousands of images that will be analyzed by local scientists.

Tucson astronomer Mike Belton, head of Galileo's imaging team, and UA planetary scientist Carolyn Porco, head of Cassini's imaging team, are in charge of the camera systems that play a large part in the Jupiter dual effort.

The spacecraft rendezvous will provide a unique opportunity that scientists hope will answer some big questions about Jupiter and its system of satellites, rings, massive magnetic field and churning atmosphere.

"It's a big bonus - never been done before," said Belton, who has been head of the Galileo imaging team since 1978.

While both spacecraft are observing Jupiter over a few months, a number of joint imaging experiments will center on Dec. 30, when Cassini makes its closest approach to the solar system's largest planet.

The Cassini team was allotted no extra funding for the Jupiter flyby, but Porco said the scientific community was "very anxious to collect data."

"It's been planned in a mad fashion."

The large amount of data being gathered is a bonus for both spacecraft. Galileo has been in orbit twice as long as expected and Cassini is passing Jupiter only for a gravity boost that will steer it on its way to Saturn, scientists said.

Galileo was launched in 1989 on a mission to Jupiter, where it has orbited since 1995. Its original mission ended in 1997. Cassini was launched in 1997 to reach Saturn in 2004. Cassini's Jupiter observation, which began in October, is really a dress rehearsal for its primary mission at Saturn.

"It's just kind of a happy coincidence that Galileo was still alive and still healthy," Porco said.

The joint imaging will take place while Cassini is at its closest approach, a distant 6 million miles away. About the same time, Galileo's orbit will swing it about 320,000 miles from Jupiter. Jupiter's distance from the sun is about 483 million miles.

For about four days, the two spacecraft will take nearly synchronized images of many aspects of the planet's system - including rings, satellites and atmosphere - with the crafts' respective advantages and disadvantages. While the resolution of Cassini's images will be moderate because of its distance, its cameras will fire off more shots than Galileo. Cassini has the ability to take several pictures consecutively, Porco said.

These images can then be converted into movies, something that's never been done before Cassini, she said. The first movies of Jupiter's churning, stormy atmosphere were completed Nov. 20, and the spacecraft will gather a total of about 25,000 pictures during the entire flyby.

Cassini also can take images with several filters, using different spectra to pick up more kinds of particles.

Galileo can't send as much data, because its main antenna never fully opened up in 1991, but the images it takes are very high in resolution. Previous images taken by Galileo of Jupiter's moons showed detailed landscapes.

Combined, astronomers will have many broad images and handfuls of intricate snapshots to compare to each other.

"You get stereo views, in a sense," said Alfred McEwen, a University of Arizona scientist who will perform a joint observation of Jupiter moons Io and Ganymede during eclipse.

Collecting images is just one of several mission objectives - the spacecraft have a combined 23 observation instruments. The largest goal is to better understand Jupiter's magnetosphere, a bubble of charged particles around the planet, by comparing Galileo's findings inside the bubble and Cassini's outside, said Scott Bolton, Jet Propulsion Lab physicist who works on both spacecraft teams.

The dual observations will end about two days after Cassini's closest approach, when the spacecraft will continue on its way toward Saturn. As it heads away from Jupiter, it will continue to image the planet's dark side, watching for lightning storms in the atmosphere, Porco said.

Galileo, however, will continue circling Jupiter until about 2002. There is a proposal before NASA to destroy the spacecraft before it runs out of fuel, by hurling it into the giant planet it has been circling since 1995.